Effective hydrometallurgical treatment of copper sulphide ores, such as chalcopyrite (CuFeS.sub.2) has been a long standing goal in the copper mining industry which has thus far eluded success. The problem lies in the fact that the severe conditions required for the effective leaching of copper from these ores results in oxidation of the sulphide in the ore or concentrate to sulphate, resulting in the generation of acid which requires expensive neutralization, rendering the process impractical and uneconomical. Attempts have been made to render the sulphide concentrate leachable under relatively milder conditions under which the sulphide would only be oxidized to elemental sulphur and not all the way through to sulphate. These attempts include the pretreatment of the concentrate prior to the pressure leaching step to render the sulphide concentrate more readily leachable, and the leaching of the concentrate in the presence of chloride ions, such as described in U.S. Pat. No. 4,039,406. In this process, the copper values in the concentrate are transformed into a solid basic copper sulphate from which the copper values must then be subsequently recovered, as described in U.S. Pat. No. 4,338,168. In the process described in U.S. Pat. No. 4,039,406 a significant amount (20-25%) of sulphide in the ore or concentrate is still oxidized to sulphate, resulting in greater oxygen demand during the pressure leach and the generation of sulphuric acid.
It is accordingly an object of the present invention to provide a hydrometallurgical copper extraction process wherein the oxidation of sulphide in the ore or concentrate to sulphate is reduced.